"Fogskum" in swedish. I don't want to take credit for coming up with this use for it though, I read somewhere about someone who made fiberglass parts for cars that used spray foam to build shapes.
Gdcast: You might want to do a little test with the foam you buy if it holds up to the resin or if it'll melt. I've heard that some brands might not work for this, but I was lucky and found one on my first try that seems to work fine. It was also the cheapest one I could find! No way?! Way! Bonus!
Oh, almost forgot; if you're planning on making shapes with bigger volumes it might be a good idea to do it in stages and let the foam cure before applying more, so that it doesn't take forever for it to dry in the middle.
Gdcast: You might want to do a little test with the foam you buy if it holds up to the resin or if it'll melt. I've heard that some brands might not work for this, but I was lucky and found one on my first try that seems to work fine. It was also the cheapest one I could find! No way?! Way! Bonus!
Oh, almost forgot; if you're planning on making shapes with bigger volumes it might be a good idea to do it in stages and let the foam cure before applying more, so that it doesn't take forever for it to dry in the middle.